Pull down resistors keep motors disabled during power-up
I see the Adafruit motor shield says this. Can I add 10K resistors to my SN754410 on pins 2 & 7 that connect to my Arduino and keep the motor from turning on at startup?
If my 2 motor control pins are 10 and 11, then I'd just add in two 10k resistors from 10 & 11 to ground? That won't interfere with the normal motor operation?
thanks
Yes. When the Arduino starts up the pins are inputs (apart from TX and pin 13) which means they will float. A pull down of 10k will be fine.
When the pins are output they can source 20 to 30mA if necessary to overcome any pull-downs so 10k is neither here or there.
However the internal pull-up resistors on the Arduino are between 20k and 50k I think. Best to have your pull-downs an order of magnitude smaller than that, namely 2k2, to ensure they always 'win' should the pins be configured as inputs with internal pull-ups. That's still only 2.3mA load.
MarkT:
Yes. When the Arduino starts up the pins are inputs (apart from TX and pin 13) which means they will float. A pull down of 10k will be fine.When the pins are output they can source 20 to 30mA if necessary to overcome any pull-downs so 10k is neither here or there.
However the internal pull-up resistors on the Arduino are between 20k and 50k I think. Best to have your pull-downs an order of magnitude smaller than that, namely 2k2, to ensure they always 'win' should the pins be configured as inputs with internal pull-ups. That's still only 2.3mA load.
...thanks, that helped me understand what's happening a little better.
Just wondering: to keep the motor from kicking when the system is powered up, could I use 1 pull-down resistor on the H bridge's enable pin, instead of 2 pull-down resisters, on each motor out pin?
I thought you WERE talking about driving the SN754410 control lines, not its motor drive output lines.
CrossRoads:
I thought you WERE talking about driving the SN754410 control lines, not its motor drive output lines.
Yes, I mis-worded my question. Let me try again: Instead of putting pulldowns on the 2 logic pins of the H-bridge that connect to the Arduino's pins, would 1 pulldown on the logic pin of the H-bridge that is the enable/disable?
on the SN754410,
1 pulldown on pin 0, instead of 2 pulldowns on pins 2 & 7
Yes, you can pull 1,2EN low instead of pulling 1A and 2A low.
You have another arduino pin available to take it high later on?
Or maybe let a capacitor charge up to enable it after power is turned on?
Yes, another I/O pin. Now that I got a Matrix Orbital LCD w/keypad controller, I have loads of pins!
thanks, again.
SouthernAtHeart:
Pull down resistors keep motors disabled during power-upI see the Adafruit motor shield says this. Can I add 10K resistors to my SN754410 on pins 2 & 7 that connect to my Arduino and keep the motor from turning on at startup?
If my 2 motor control pins are 10 and 11, then I'd just add in two 10k resistors from 10 & 11 to ground? That won't interfere with the normal motor operation?
thanks
NOTE!! I would just like to point out that after diving into the datasheet for the SN754410E QUAD H-Bridge Motor Driver Chip, I found that it states the following on the first page, "No Output Glitch During Power Up or Power Down".
When looking further into the datasheet to see what that meant, I found the following:
(See datasheet: figure 3) http://www.jameco.com/Jameco/Products/ProdDS/1054684.pdf
Figure 3 is a diagram of the internal logic for the SN754410E Quad H-Bridge chip, showing there is already an INTERNAL 10K OHM RESISTOR connecting the +5V input to the two (1,2 EN and 3,4 EN) enable pins. So this problem looks like it was already taken care of. I did find that creating a breadboard test circuit for a self-balancing robot with an Arduino UNO R3, SN754410E motor driver chip, and 7805T voltage regulator circuit for an external DC power source, yielded results showing that there is no motor glitching during the power up or power down of the Arduino.
I hope this helps someone rest assured!